The intensity of the radiation beam used for radiotherapy is required to be time-invariant in some applications or time-varying in other applications. Temporal variation of beam intensity provides additional degree of freedom for stereotactic radiotherapy where beam apertures are varied with respect to gantry angle and/or desired segment. Variation of beam apertures and associated beam intensities can be done while the gantry is either rotating or stationary at a sequence of gantry orientations. The former is called Intensity modulated arc therapy (IMAT) and the latter is called “step-and-shoot” Intensity Modulated Radiation therapy (IMRT).